Conventional Technology
Recently, electronics devices are becoming smaller and smaller. As representatively found in notebook computers, word processors, portable telephones and personal digital assistants, liquid crystal displays which are advantageously light and compact are often used in such electronics devices. In these liquid crystal display devices, various films (e.g., a polarization film) are used to ensure display quality. In applications such as personal digital assistants or portable telephones, a plastic liquid crystal display device employing a resin film in place of a glass substrate is practically used.
A resin film used in a device handling with polarized light, such as a liquid crystal display device, needs to be not only optically transparent but also optically uniform. In the case of a film substrate used in a plastic liquid crystal display device, a retardation represented by a product of the birefringence and thickness of the film substrate not only needs to be small, but also needs to be unlikely to be changed even when external stress is applied.
In the case of a resin film, it has been found that polarization and alignment of resin molecules in the film involve a retardation. To obtain a film having a small retardation requires the use of a resin having small polarization. Further, conditions under which such a film is produced need to be regulated in such a manner as to suppress alignment of molecules as much as possible.
In general, engineering plastic resins such as polycarbonate, polyarylate, polysulfone, and polyethersulfone, and celluloses such as triacetylcellulose are known as resin for film. When these resins are used to produce a film, a variety of stress is applied on a film being formed. Such stress is caused by: a back pressure used to flow a melted resin; shrinkage of the resin as the solvent is dried or heat shrinkage of the resin; the tension of the film as it is being conveyed; or the like. The stress causes the molecules in the film to be aligned, so that a retardation is very likely to remain in the film.
To solve the above-described problems, an attempt has been made to use a resin having small polarization to obtain a film. For example, an attempt has been made to use an olefin resin such as, typically, a cycloolefin resin to produce a film.
The range of applications of retardation films is becoming more and more widespread. A higher level of performance is required for the retardation films as the films are more and more widely used. The wavelength dependence of a retardation has received attention as a particularly important feature. The wavelength dependence of a retardation is generally defined as Re(400)/Re(550) where Re(400) represents a retardation of a film measured using monochromatic light having a wavelength of 400 nm, and Re(550) represents a retardation of the film measured using monochromatic light having a wavelength of 550 nm. A value defined by Re(400)/Re(550) is herein called wavelength dependence unless otherwise specified.
In an STN liquid crystal display device, for example, an appropriate wavelength dependence is desired for a retardation film for use in color compensation.
On the other hand, a film used in a quarter-wave plate or a half-wave plate is desired to have a retardation corresponding to ¼ or ½ of wavelength with respect to all the wavelengths of visible light. However, a conventional retardation film of polycarbonate has a large wavelength dependence of 1.16 and a large alignment retardation (described later) of 1400. In other words, the magnitude of polarization varies depending on wavelength. Assuming that such a retardation film having a large wavelength dependence is used in a reflection TFT liquid crystal display device, contrast is significantly deteriorated when a black display is performed in the liquid crystal display device.
Problems to be Solved by the Invention
A film of an engineering plastic resin, such as the above-described polycarbonate, has a retardation. Therefore, a remaining retardation needs to be reduced by providing a special process such as heating and annealing such a film.
Even when a film having a reduced retardation is produced in this way, a retardation is often exhibited again since subsequent handling with the film leads to alignment of molecules. For example, when the film and a polarization plate are laminated to each other, the polarization plate is often deformed. If the polarization plate is deformed, stress is generated. The stress causes alignment of molecules, leading to exhibition of a retardation. Therefore, the above-described film needs to be carefully handled. Despite the careful handling, the yield (i.e., the probability that an end product having a small retardation is obtained) is disadvantageously low.
In particular, it is known that when the above-described film is used as a polarizer protection film, the stress as the polarizer shrinks causes exhibition of a retardation which is undesirable for a film. Such a retardation has an adverse influence on the polarization performance of a polarization film.
The above-described film is produced by various production methods. One of the methods is a solvent cast method. A film produced by the solvent cast method has a relatively small retardation in a plane of the film. However, the film produced by the solvent cast method has a large retardation in a thickness direction of the film due to birefringence generated by alignment of molecules, so that the viewing angle characteristic of the film is likely to be adversely reduced.
Alternatively, a retardation film may be obtained by stretching. When a retardation film is obtained by stretching, a retardation varies due to a small variation in tension generated by a stretching device. Moreover, when the retardation film obtained by stretching is laminated to a polarizer or the like, a retardation is likely to be changed due to tension generated by the lamination. Thus, the desired value is unlikely to be maintained. Moreover, stress is likely to be generated due to, for example, shrinkage of the polarization plate after the lamination so that a retardation value is likely to be adversely changed.
Further, the wavelength dependence of a retardation is dependent only on the materials of the retardation film. The required wavelength dependence differs from application to application. To obtain different wavelength dependence, a different material needed to be selected. Therefore, a new material disadvantageously needed to be found for an application requiring a different wavelength dependence.
Furthermore, the above-described film is often laminated to a glass or another film. However, in the case of a film made of a resin having small polarization, such as olefin type resin, such a film has a poor adhesion strength to glues or adhesives. Therefore, a special adhesive is often required for the film. Moreover, surface treatment is often required for the film.